The invention resides in a semi-permeable membrane for magnetic particle fractions in a fluid having two membrane surface forming a separation structure between two fluid fractions and at least one channel bridging the membrane.
A membrane of this type is in contrast to vibrating membranes—a separation membrane—which forms a separation layer between two fluid volumes. A semi-permeable (osmotic) membrane is only permeable by certain materials or components of a fluid whereas, it forms an impermeable barrier for other materials. This selective material exchange, also called osmosis, can occur in both directions (diosmosis) or, as in the present case, in one direction (endosmosis).
Semi-permeable membranes are, for example, necessary components for life. Each biological cell is surrounded by a semi-permeable membrane, that is, a membrane which is permeable only for certain substances.
Semi-permeable membranes are also suitable for technical separation methods. An overview over membrane methods as well as over membrane methods with semi-permeable membranes is provided in chapter “Membranes and Membrane Separation Process”, Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH Verlag, 2002.
An embodiment as shown in this publication which is used in electrodialysis procedures is based on a combination of an electric field and a semi-permeable membrane. With the application of a magnetic field, a flow of ions through the semi-permeable ion exchanger membrane in a certain direction is established. These membranes ideally permit the passage of ions of a certain charge type whereas ions of the opposite charge type are prevented from passing. For example, cat-ions may pass a cat-ion exchanger membrane whereas an-ions are fully blocked. By a combination of a driving force (electric field, pressure, etc. . . . ) and a semi-permeable membrane, a continuous separation of a substance against a concentration differential is therefore possible.
Further semi-permeable membranes for endosmotic methods are based on the utilization of hydrophilic and hydrophobic material properties and concern for example the selective removal of organic compounds from aqueous solutions via so-called per-vaporation membranes. The organic compounds pass through the hydrophobic membrane from one side thereof and are vaporized on the other side of the membrane whereas water is blocked from passing through the membrane.
The selection features of the materials to be separated are based therefore on the electric charges (positive or negative) or hydrophilic or hydrophobic properties, which limits the use possibilities of a technical endosmosis to certain material groups.
In order to expand the apparent selectivity of a membrane the above publication proposes a combination of this substance with another substance or the bonding of this substance to a carrier particle. For example, charge-free molecules can be prepared by a chemical reaction with a charged molecule for an endosmosis based on the mentioned electrostatic effects. On the other hand, the bonding of substances to macromolecules or particles is often utilized to make the larger bond structure formed in this way impassable for membranes which are only permeable for small particle sizes (Nano-, ultra-, or ultrafiltrations) and to concentrate them whereas the surrounding solution passes through the membrane because of a pressure gradient. However, the last example differs from the previously described membrane method in that the desired substance is retained and does not pass the membrane that is the desired substance remains in the original solution medium, though in concentrated from. In the method with a semi-permeable membrane, in contrast, the desired substance passes through the membrane and into the medium disposed at the other side of the membrane. The medium at the permeate side of the membrane may differ from the medium at the feed or, respectively, the retentate side, for example, in the pH value and the salt content or it may consist of a completely different solvent such as, for example, ethanol. With pervaporation, even a change-over from a liquid to a gaseous medium occurs. The medium change-over provides for substantially wider chemical engineering application possibilities than simple concentrating via size-selective membranes.
More diverse than size-selective membranes are functionalized membranes which are solid phases in membrane form which are effective by adsorption. For this purpose, polymer membranes, which are effective on the basis of a defined pore distribution by size exclusion are functionalized in a further step. Another possibility resides, for example, in pressing or weaving fiber-like sorption materials into membrane form. If a medium then flows through these membranes, certain substances are retained by sorption by the functional groups. The membranes act in principle like a very flat sorption filter. As a result of a large flow cross-section which can be provided thereby, a sufficient flow volume can be obtained even at low flow speeds. However, with the sorption principle the binding capacity limit is reached relatively rapidly and the flow must then be stopped to avoid a breakthrough. Subsequently, the sorptive membrane must be regenerated in a separate step which occurs generally by elution of the previously retained substances. In this way, the substances can be transferred—like with regular semi-permeable membranes—to a different solvent. However, in contrast to the continuous method of regular semi-permeable membranes, membranes operating on a sorption basis require a cyclic discontinuous operation.
If it would be possible to provide for a selection of permeable and non-permeable substances in a much more variable and specific way, a wide application area would be available in the chemical and bio-technical industry.
It is therefore the object of the present invention to provide a semi-permeable membrane system which can be used for an endosmosis for magnetic particle fractions, that is, which acts as a semi-permeable barrier in connection with magnetic particles. This system is intended to provide for a continuous passage of magnetic particles through the membrane so that the magnetic particles travel from one fluid at one side of the membrane to another fluid at the opposite side of the membrane.